Induction motors are considered asynchronous. This is because while their speed depends upon the external sine wave, it is not precisely synchronized to it.
In an induction motor, the ac power supply connects directly to the field coils that are part of the stator. The alternating power produces a rotating magnetic field. This field induces current in the rotor and a corresponding rotating magnetic field by way of mutual induction. Thus the stator and rotor are in effect the primary and secondary of a transformer.
In dc and other synchronous motors, one magnetic field is static while the other rotates by means of either internal or external commutation. In the induction motor, both fields rotate. But the interaction that will rotate the motor shaft will not take place if both fields rotate at the same speed. Motion of one magnetic field with respect to the other is what makes mechanical energy output possible in a motor. In an induction motor, the rotating magnetic field (and hence rotor) speed of rotation is slightly less than the stator’s magnetic field speed of rotation. The difference of the two speeds, expressed as a percentage, is known as “slip”. Slip should not be viewed as some wasted energy caused by inefficient operation or a design defect, but rather as a necessary property of the induction motor, without which it would be dead in the water.
The windings in the rotor of an induction motor have low resistance. A small slip induces high rotor current and torque. When the motor starts rotating the slip is 100% and the motor current is at maximum. The slip and motor current drop as the rotor starts to turn. A small induction motor may have over 5% slip at full load while in larger induction motors the full-load slip may be under 1%.
Because there are no brushes or moving parts in an induction motor other than the rotor, supported by usually sealed bearings, the induction motor is robust and maintenance free. Polyphase induction motors in many sizes and designs power industrial installations throughout the world. In the home, induction motors power fans, appliances and larger tools such as table saws where universal motors are not used.
Nikola Tesla and Galileo Ferraris appear to have independently invented and built working models of the induction motor in the 1880’s. Tesla seems to have won the battle of the patents, but George Westinghouse covered all bets, buying an option to Ferraris’s U.S. rights and licensing Tesla’s patent. He employed the creative genius as a consultant as well. Eventually, Westinghouse produced highly successful induction motors, which went on to gain favor in farm, factory and home throughout the world.
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